1. Field of the Invention
The present invention relates to a dip coating apparatus, and particularly to a dip coating apparatus in which a substrate which is supported by a supporter having a gas discharger is dipped into a coating liquid to form a coating layer on the surface of the substrate. More particularly, the present invention relates to a dip coating apparatus suitable for forming a cylindrical photoconductor.
2. Discussion of the Background
As a typical coating method for forming a photoconductive layer on the peripheral surface of a cylindrical substrate, dip coating methods have been widely used. However, the dip coating methods have a drawback in that the thickness of a coated layer formed near a coat-starting end, i.e., an upper coating end, of the substrate is thinner than the other areas of a coated layer formed on the substrate, because the coated liquid on the upper part tends to drop downward during the drying time of the coated layer. This phenomenon is called as "a liquid dropping problem". In attempting to solve the liquid dropping problem, various dip coating methods have been disclosed. However, these methods have drawbacks in that the coating apparatus are complicated, the thickness of the coated layer is uneven, and/or the effect of the improvement is insufficient.
For example, Japanese Laid-Open Patent Publication No. 59-127,049 discloses dip coating methods in which, when a cylindrical substrate is pulled from a coating liquid, the density of vapor, which is generated by the vaporization of a solvent included in the coated liquid, is reduced. However, this coating method has following drawbacks:
(1) a gas passage from which the vapor is removed must be disposed in the upper part of the coating vessel, resulting in complications in operation of the coating apparatus; and PA1 (2) since the direction of the airflow is vertical to a surface of the cylindrical substrate, the thickness of the areas of the coated layer against which air is blown becomes thinner than the other areas of the coated layer formed on the substrate. PA1 a substrate supporter which supports a substrate to be coated and which has a gas discharger which has a plurality of holes formed therein from which a gas is discharged; PA1 a gas supplying passage through which the gas is supplied to the gas discharger; and PA1 a gas supplying valve which is disposed at a position in the gas supplying passage and which feeds the gas to the gas discharger when the gas supplying valve is opened, PA1 a plurality of substrate supporters, each of which supports a substrate to be coated and which has a gas discharger which has a plurality of holes formed therein from which a gas is discharged; PA1 a gas supplying passage which is branched from a branching point and through which the gas is supplied to each the gas discharger; and PA1 a gas supplying valve which is disposed at a position in the gas supplying passage, which is upstream from the branching point, and which valve feeds the gas to each the gas discharger when the valve is opened,
Japanese Laid-Open Patent Publication No. 59-225771 discloses a dip coating method in which when a cylindrical substrate is pulled from the coating liquid, a ring-shaped air control member blows air against the cylindrical substrate. However, the coating method also has a drawback in that the thickness of the formed layer is uneven because airflow in the coating vessel is uneven and the air which is discharged from the ring-shaped air doctor directly blows against the coated layer.
Japanese Laid-Open Patent Publication No. 63-7873 discloses a coating method in which a hood which is extendible and contractible is disposed in an upper part of the coating vessel to reduce the density of the solvent vapor. However, he coating apparatus becomes complicated, and in addition the apparatus is not practical because it takes a long time to change the substrate and the coating liquid.
Japanese Laid-Open Patent Publication No. 1-107874 discloses a coating method in which when a substrate is pulled from a coating liquid, an airflow is generated in a horizontal direction. This method also has a drawback in that the formed layer is uneven, which is the same drawback as that of the method disclosed in Japanese Laid-Open Patent Publication No. 59-127049 mentioned above.
Japanese Laid-Open Patent Publication No. 3-213171 discloses a coating method in which when a substrate is pulled from a coating liquid, air is blown against the peripheral surface of the cylindrical substrate while the air rotates around the surface. However, the method is complicated and the formed layer becomes uneven because air directly blows against the coated layer.
Japanese Laid-Open Patent Publication No. 4-29773 discloses a coating method in which when a substrate is pulled from a coating liquid, air which is discharged from nozzles blows against the substrate in a direction of the tangent line of the cylindrical substrate. However, the formed layer is uneven because the areas of the coated layer against which air is blown have less thickness than the other areas of the coated layer.
In addition, in attempting to solve the liquid dropping problem, for example, Japanese Laid-Open Patent Publications Nos. 5-7812 and 5-88385 have disclosed coating methods. However, these methods also have the drawbacks in that the apparatus are complicated and a resultant coated layer tends to be uneven.
Further, halogen-containing solvents such as methylene chloride have been typically used for the charge transporting layer coating liquid. However, currently the halogen-containing solvents cannot be used or are regulated in order to avoid environmental pollution and to prevent the ozone layer from being damaged. When other solvents are used as the solvents of the coating liquids, the liquid dropping problem tends to occur more frequently because the evaporating speed of the solvents is generally slower than that of the halogen-containing solvents.
In addition, currently polymers which have good abrasion resistance are used as binder resins of photoconductive layers, or polymer charge transporting materials are used in charge transporting layers, in order to prolong the life of photoconductors. In these cases, the coating liquids have higher viscosity than ever, and therefore the coating liquids are diluted when the liquids are coated, resulting in frequent occurrence of the liquid dropping problem.
In attempting to solve these problems and to form a photoconductor having a uniform coated layer, the present inventor and another inventor propose in Japanese Laid-Open Patent Publication No.9-265193 a dip coating apparatus in which a substrate which is supported from the inside thereof by a substrate supporter having a gas discharger which is cylindrical and which is porous is dipped into a coating liquid to form a uniform coating layer on the substrate, wherein a gas discharged from the gas discharger blows downward against the surface of the coating liquid. However, when a photoconductor is manufactured even by this coating apparatus, the photoconductive layer formed on a substrate sometimes has uneven thickness. In addition, when a plurality of photoconductors are manufactured at the same time using this coating apparatus, a problem which occurs is that each thickness of the resultant photoconductive layers is different from each other.
Because of these reasons, a need exists for a dip coating apparatus by which a plurality of photoconductors having a uniform photoconductive layer can be manufactured at the same time.